Question

Consider the possibilities for stereoisomerism in the bromohydrin and epoxide formed from trans-2-butene. (a) How many stereoisomers are possible for the bromohydrin? Which of the possible bromohydrin stereoisomers are formed by treating trans-2-butene with bromine in water? (b) How many stereoisomers are possible for the epoxide? Which of the possible stereoisomers is/are formed in this two-step sequence?

Short answer

Answer: There are 4 possible stereoisomers of bromohydrin and epoxide that can be formed from trans-2-butene. When treated with specific reagents, the possible bromohydrin stereoisomers formed are (2R,3R)-2-bromo-3-hydroxybutane and (2S,3S)-2-bromo-3-hydroxybutane. In a two-step sequence, the possible epoxide stereoisomers formed are (2R,3R)-2,3-epoxybutane and (2S,3S)-2,3-epoxybutane.

Step-by-step solution

(a) Step 1: Draw the Lewis Structure of Trans-2-butene

(Draw the Lewis structure of trans-2-butene, which is an alkene with 4 carbons and a double bond between C2 and C3, with two methyl groups on opposite sides of the double bond. The molecular formula is CH_3CH=CHCH_3).

(a) Step 2: Identify the Reagents for Bromohydrin Formation

(When trans-2-butene is treated with bromine in water (Br_2 and H_2O), the double bond is broken, and a bromine atom is added to one carbon on the double bond, and a hydroxy group (OH) is added to the other carbon, forming bromohydrin).

(a) Step 3: Formation of Bromohydrin Stereocenters

(At the carbons where the bromine and hydroxy group are added, we will have two stereocenters. For each of these stereocenters, there are two possible configurations (R or S). This gives us a total of 2^2 = 4 possible stereoisomers for bromohydrin).

(a) Step 4: Determining the Bromohydrin Isomers formed from Trans-2-butene

(When trans-2-butene reacts with bromine in water, we get a syn addition, meaning that the OH and Br groups will be added to the same side of the molecule. This syn addition will result in the formation of two possible bromohydrin stereoisomers (RR and SS stereoisomers). Thus, the possible bromohydrin stereoisomers formed are (2R,3R)-2-bromo-3-hydroxybutane and (2S,3S)-2-bromo-3-hydroxybutane).

(b) Step 1: Formation of Epoxide from Bromohydrin

(To form the epoxide, the hydroxy group (OH) from the bromohydrin reacts with the adjacent carbon containing the bromine, causing the bromine to leave, and forming a new oxygen-carbon bond. This results in a three-membered ring called epoxide).

(b) Step 2: Identify Stereocenters in the Epoxide

(When the epoxide is formed, the initial stereocenters do not change their configuration. Therefore, the number of stereoisomers for the epoxide formed from bromohydrin remains the same, which is 4 possible stereoisomers).

(b) Step 3: Determining the Epoxide Stereoisomer(s) Formed in the Two-Step Sequence

(Since we are forming epoxide from the two possible bromohydrin stereoisomers obtained by treating trans-2-butene with Br_2 and H_2O, the epoxide stereoisomers that can be formed are those with the same RR and SS configuration as the bromohydrin. Thus, the possible epoxide stereoisomers formed in this two-step sequence are (2R,3R)-2,3-epoxybutane and (2S,3S)-2,3-epoxybutane).